1. Technical Field of the Invention
The present invention relates generally to an improved fabrication method of a metal shell installed on a spark plug which may be employed in automotive internal combustion engines.
2. Background Art
Typical plug metal shells are installed on spark plugs by staking an annular wrapping end of the metal shell on a porcelain insulator of the spark plug. The wrapping end of the metal shell is usually made by cold forging. The metal shell also has a hollow cylindrical base portion and a hexagonal boss which are also shaped by the cold forging. The hollow cylindrical base portion has threads formed in an exterior surface thereof by rolling.
FIG. 6 illustrates a conventional forging process for fabricating a metal shell of a spark plug, as disclosed in Japanese Patent First Publication No. 7-16693, which forms a wrapping end 11 and a small-diameter base portion 12 of the metal shell in a single process. The formation of the wrapping end 11 is accomplished by striking a large-diameter head portion 13 of a hollow cylindrical workpiece with a cylindrical punch 50 to decrease the diameter of the head portion 13. An outer wall of the small-diameter base portion 12 is shaped by a die 52.
The simultaneous formation of the wrapping end 11 and the small-diameter base portion requires a punch holder 51. It is impossible for the punch holder 51 to have an outer diameter greater than that of the large-diameter head portion of the workpiece. The punch holder 51 must, therefore, be formed to be thin, so that it has a low strength. Forging the workpiece requires exertion of a large pressure on the punch holder 51, which will lead to a problem that cracks or physical deformation of the punch holder 51 arise within a short period of time.
Further, it is difficult to form a large rounded inner wall in an end of the punch holder 51 because it is thin, which results in a drop in fluidity of material of the workpiece around the end of the punch holder 51. This causes a boundary between inner walls of the wrapping end 11 and the large-diameter head portion 13 to be subjected to shrinkage, which may result in formation of cracks near the boundary between the wrapping end 11 and the large-diameter head portion 13.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a fabrication method for fabricating a metal shell which is less susceptible to cracks when installed on a spark plug and has an increased service life.
According to one aspect of the invention, there is provided an improved fabrication method of a metal shell to be installed on a park plug which may be employed in automotive engines. The metal shell has a given length and is made up of a small-diameter portion, a large-diameter portion, and a wrapping portion. The wrapping portion is to be wrapped by staking about a porcelain insulator of a spark plug to achieve installation of the metal shell on the spark plug. The method comprises the steps of: (a) preparing a cylindrical workpiece which has a given length with a first and a second end opposed to each other; (b) preparing a punch and a die; (c) placing the workpiece in the die and pressing the workpiece with the punch from the second end of the workpiece to shape the wrapping portion of the metal shell on a side of the first end of the workpiece in a first cold forging process; and (d) processing the workpiece to shape the small-diameter portion of the metal shell on a side of the second end of the workpiece in a second cold forging process.
In the preferred mode of the invention, the method further comprises the step of forming threads on an outer peripheral wall of the small-diameter portion for installation of the spark plug.
The method further comprises the step of processing the workpiece to form a large-diameter portion on the side of the second end and a small-diameter portion on the side of the first end prior to the first cold forging process in which the wrapping portion is formed.
In the first cold forging process, a portion of the workpiece on the side of the first end is pressed within the die stepwise to decrease, in sequence, the portion of the workpiece in outer diameter to shape the wrapping portion of the metal shell.
A hexagonal boss may be formed on the large-diameter portion of the workpiece in the first cold forging process.
The hexagonal boss may alternatively be formed on the large-diameter portion of the workpiece in a third process different from the first and second cold forging process.